Method of fabricating an air-gap spacer of a metal-oxide-semiconductor device

ABSTRACT

A method of fabricating an air-gap spacer of a metal-oxide-semiconductor device includes the following steps. First, a substrate having a gate oxide layer and a polysilicon layer successively formed is provided. The polysilicon layer and the gate oxide layer are patterned to form a gate electrode region. A silicon nitride layer and an oxide layer are successively formed on the surface of the substrate and the surface of the gate electrode region. The oxide layer and the silicon nitride layer are anisotropically etched to form a cross-sectional L-shaped silicon nitride layer and a first spacer at the sidewall of the gate electrode region. After the first spacer is removed, an ion implantation is performed to form an extended lightly doped region below the L-shaped silicon nitride layer in the substrate and a lightly doped region in the substrate surrounding the extended lightly doped region. A second spacer is formed at the sidewall of the L-shaped silicon nitride layer wherein the second spacer covers the L-shaped silicon nitride layer. An ion implantation process is performed to form source/drain regions, using the second spacer and the gate electrode region as masks. The L-shaped silicon nitride layer is removed to form an L-shaped air-gap region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to the fabrication of a metal-oxide-semiconductor device (MOS), and more particularly to the method of forming a MOS device, having an air-gap between the gate electrode and the spacer.

2. Description of the Related Art

In order to avoid short-channel effect, lightly doped drain (LDD) structure is frequently utilized in a MOS device. Referring to FIG. 1, LDD structure 11 contains a lightly doped region located at a region between the source/drain region 15 and the gate electrode 12 at the substrate 10 surface. However, the existence of the spacer 13 between the gate electrode 12 and the LDD region 11 results in an overlap capacitor 14, which is easy to cause the misjudgement during data access.

To reduce the overlap capacitor forming between the gate electrode region and LDD region, material with lower dielectric constant is frequently utilized as spacer. Besides, a MOS having an air-gap region between the gate electrode region and the spacer is proposed to prevent the formation of the overlap capacitor. Referring to FIG. 2, a MOS device 20 includes a gate electrode region 21, source/drain regions 22, a LDD region 23 and a spacer 24. The spacer 24 and the gate electrode region 21 are not directly connected but isolated by an air gap region 25. Therefore, the overlap capacitor between the gate electrode region 21 and the LDD region 23 is reduced.

However, the MOS device with the air-gap structure can not completely avoids the formation of the overlap capacitor since the spacer partially contacts with the LDD region.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a process for fabricating a MOS device having an L-shaped air-gap region extending onto the surface of the LDD region to reduce the overlap capacitor between the gate electrode and the LDD region.

It therefore another object of the invention to provide a method of fabricating a MOS device having an air-gap spacer, wherein a more lightly doped drain can be formed and the short channel effect is avoided.

A method of fabricating an air-gap spacer of a metal-oxide-semiconductor device includes the following steps. First, a substrate having a gate oxide layer and a polysilicon layer successively formed is provided. The polysilicon layer and the gate oxide layer are patterned to form a gate electrode region. A silicon nitride layer and an oxide layer are successively formed on the surface of the substrate and the surface of the gate electrode region. The oxide layer and the silicon nitride layer are anisotropically etched to form a cross-sectional L-shaped silicon nitride layer and a first spacer at the sidewall of the gate electrode region. After the first spacer is removed, an ion implantation is performed to form an extended lightly doped region below the L-shaped silicon nitride layer in the substrate and a lightly doped region in the substrate surrounding the extended lightly doped region. A second spacer is formed at the sidewall of the L-shaped silicon nitride layer wherein the second spacer covers the L-shaped silicon nitride layer. An ion implantation process is performed to form source/drain regions, using the second spacer and the gate electrode region as masks. The L-shaped silicon nitride layer is removed to form an L-shaped air-gap region.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The description is made with reference to the accompanying drawings in which:

FIGS. 1 (Prior Art) is a cross-sectional view showing a conventional MOS device.

FIGS. 2 (Prior Art) is a cross-sectional view showing a conventional MOS device having an air-gap spacer.

FIGS. 3A to 3G are cross-sectional views showing the process steps of forming a MOS device having an air-gap spacer according to a preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 3A, on a substrate 30, a gate oxide layer 31, a polysilicon layer 32, and an anti-reflection layer 33 are successively formed. The anti-reflection layer 33 can be silicon nitrous oxide (SiNO) which benefits the control of the critical dimension (CD) during the further photolithography process. The anti-reflection layer 33, the polysilicon layer 32, and the gate oxide layer 31 are then patterned to form a gate electrode region 34 shown in FIG. 3B.

Then, referring to FIG. 3C, a silicon nitride layer 35 is formed on the surface of the substrate 30 and the side exposed surface of the gate electrode region 34, preferably by chemical vapor deposition. Next, an oxide layer 36, such as chemical vapor deposited silicon dioxide, is formed on the surface of the silicon nitride layer 35. Then, the oxide layer 36 and the silicon nitride layer 35 are anisotropically etched back to form a spacer 37 at the sidewall of the gate electrode region 34 and a cross-sectional L-shaped silicon nitride layer 35a between the spacer 37 and the gate electrode region 34 shown in FIG. 3D.

Referring to FIG. 3E, after the spacer 37 is removed, the substrate 30 is process through ion implantation. Consequently, the substrate region covered by the L-shaped silicon nitride layer forms a shallow and more lightly doped diffusion region 38 and the exposed substrate surface forms a deep and less lightly doped diffusion region 39.

Then, referring to FIG. 3F, a spacer 40, preferably an oxide spacer, is formed at the sidewall of the gate electrode region 34 wherein the spacer 40 must be able to cover the L-shaped silicon nitride layer 35a. The process steps of fabricating the spacer 40 can be similar to the ones of fabricating the spacer 37. Next, the substrate 30 is processed through ion implantation, using the spacer 40 and the gate electrode region 34 as masks, to form the source/drain regions 41.

Referring now to FIG. 3G, the L-shaped silicon nitride layer 35a and the anti-reflection layer 33 are removed, using such as hot phosphoric acid solution. Therefore, the L-shaped air-gap region 42 is formed between the gate electrode region 34 and the spacer 40. By now the MOS device with an L-shaped air-gap between the gate electrode region 34 and the spacer 40 is formed. Since the back-end processes are not closely related to the subject of the invention, they will not be described herein.

Using the above-mentioned method forms a MOS device with an L-shaped air-gap. Besides, the source/drain structure is elevated to include two lightly doped regions, one region 38 more lightly doped and closer to the gate electrode region 34 than the other 39. As a result, the overlap capacitor between the source/drain regions is reduced, the short channel effect is improved and the phenomena of punch through is avoided.

While the invention has been described by way of example and terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures. 

What is claimed is:
 1. A method of fabricating an air-gap spacer of a metal-oxide-semiconductor device, comprising the steps of:providing a substrate, wherein a gate oxide layer and a polysilicon layer are successively formed on the substrate; patterning the polysilicon layer and the gate oxide layer to form a gate electrode region; forming a silicon nitride layer and an oxide layer successively on a surface of the substrate and a surface of the gate electrode region; anisotropically etching the oxide layer and the silicon nitride layer to form a cross-sectional L-shaped silicon nitride layer and a first spacer at the sidewall of the gate electrode region; removing the first spacer; performing ion implantation to form an extended lightly doped region below the L-shaped silicon nitride layer in the substrate and a lightly doped region in the substrate surrounding the extended lightly doped region; forming a second spacer at the sidewall of the L-shaped silicon nitride layer wherein the second spacer covers the L-shaped silicon nitride layer; performing an ion implantation process to the substrate to form source/drain regions, using the second spacer and the gate electrode region as masks; and removing the L-shaped silicon nitride layer to form an L-shaped air-gap region.
 2. A method as claimed in claim 1, wherein an antireflection layer can be further formed over the polysilicon layer.
 3. A method as claimed in claim 2, wherein the antireflection layer can be silicon nitrous oxide.
 4. A method as claimed in claim 1, wherein the silicon nitride layer is formed by chemical vapor deposition.
 5. A method as claimed in claim 1, wherein the oxide layer is a silicon dioxide layer formed by chemical vapor deposition.
 6. A method as claimed in claim 1, wherein the extended lightly doped region is shallower than the lightly doped region.
 7. A method as claimed in claim 1, wherein the second spacer includes silicon dioxide.
 8. A method as claimed in claim 1, wherein the L-shaped silicon nitride layer is removed by hot phosphoric acid solution. 